Usually, a multitone method (DMT—discrete multitone) is used for asymmetric data stream transmission via normal telephone lines, normal telephone lines usually being constructed as asymmetric digital subscriber lines (ADSL).
An essential advantage of ADSL transmission techniques consists in being able to use conventional cable networks for a transmission, twisted copper conductors normally being used.
High-speed digital subscriber lines of the prior art are described, for example, in the publication “High-speed digital subscriber lines, IEEE Journal Sel. Ar. In Comm., Vol. 9, No. 6, August 1991”.
Among the transmission methods with a high data rate, which are based on digital subscriber lines (DSL), a number of VDSL (Very High Data Rate DSL) arrangements are known and, for example, methods such as carrierless amplitude/phase (CAP), discrete wavelet multitone (DWMT), single line code (SLC) and discrete multitone (DMT) can be used for these. In the DMT method, the transmit signal is provided from multiple sinusoidal or cosinusoidal signals, where both the amplitude and the phase can be modulated of each individual sinusoidal or cosinusoidal signal. The multiple modulated signals thus obtained are provided as quadrature-amplitude modulated (QAM) signals.
FIG. 4 shows a conventional data stream receiver for receiving an analog data stream 101 which contains multitone signals. The multitone signals are provided by a data stream transmitter and transmitted via a transmission channel as will be described in greater detail below. After the analog data stream 101 has been received in a preprocessing device 301, a preprocessed digital data stream 302 is provided for further processing.
The preprocessing device 301 contains in conventional manner an analog/digital converter 104 by means of which the analog data stream 101 is converted into a digital data stream 103. The digital data stream 103 is then converted in conventional manner into a filtered data stream by means of a first filtering device 401, the first filtering device 401 providing a decimation of the incoming digital data stream 103.
The data thus decimated and filtered by the first filtering device 401 are provided to a second filtering device 402 in which a time domain equalization is carried out. The second filtering device 402 is constructed as an adaptive transversal filter which operates at a symbol sampling rate Fs which is, for example, 276 kHz with ADSL at a switching center. The signal equalized by the second filtering device 402 is supplied as a preprocessed digital data stream 302 to a transformation device 110 in which, for example, a fast Fourier transformation (FFT) is carried out.
The transformation signals 111a–111n formed as a complex number which is defined, for example, in accordance with amount and phase, are then supplied to a correction device 112 in which a correction of a transfer characteristic of the transmission channel is provided. The corrected transformation signals 113a–113n are also supplied to a determining device 116 in which pairs of amount signals 114 and phase signals 115 are determined in accordance with the multitone signals in the analog data stream 101. The pairs of amount signals 114 and phase signals 115 are supplied to a decoding device 117 in which the pairs of amount signals and phase signals are decoded in a decoder data stream 118. The decoded data stream 118 is then output via a data output device 119.
The frequencies of the multitone signal contained in the analog data stream 101 to be transmitted are usually equidistantly distributed and become calculable in accordance with the following formula:
            f      l        =                            i          ·                      1            T                          ⁢                                  ⁢        i            =      1        ,  2  ,      …    ⁢                  ⁢          N      /      2      where T is a period and N is a number of samples of a DMT symbol.
For example, conventional DMT methods use 256 tones which can be modulated in amount and phase in each case as sinusoidal tones. The fundamental frequency is 4.3 kHz and the frequency spacing between successive tones is also 4.3 kHz. Thus, a frequency spectrum from 4.3 kHz (fundamental frequency) to (4.3 kHz+256×4.3 kHz)=1.1 MHz is transmitted. Each DMT symbol is thus represented by a sinusoidal tone which can be modulated in amount and phase, a maximum of 15 bits per symbol usually being represented as complex number. During the transmission of a multitone signal of this type, the problem occurs, however, that transient effects are produced by the transmission channel which, for example, can be constructed as a twisted copper dual wire, which effects have decayed after, for example, M samples.
In the transmitter device, the last M samples of a DMT symbol are appended to a block start after an inverse fast Fourier transformation (IFFT), where the following relation applies: M<N. Due to this cyclic extension (cyclic prefix), a periodic signal can be simulated for the data stream receiver when the transient effect caused by the transmission channel has decayed after M samples and mutual interference between different DMT symbols, i.e. inter-symbol interference (ISI), can be avoided.
As a result, an equalization effort in an equalizer arranged in the data stream receiver can be considerably reduced in conventional methods since after demodulation of the received analog data stream 101 in the data stream receiver, only a simple correction with the inverse frequency response of the transmission channel must be performed in the correction device 112.
In methods according to the prior art, identification of a transmission channel is provided by a transfer function which is given by the following equation:H(z)=B(z)/A(z).
An equalizer is conventionally adjusted in such a manner that a cascading of the channel transfer function of the transmission channel and of the transfer function of the equalizer provides a resultant transfer function Hr as follows:Hr=B(z).
It can be clearly seen that a length of a remaining impulse response is thus determined by the order of the numerator polynomial B(z).
In the known methods described above, the equalizer operates at a sampling rate Fs which is, for example, Fs=276 kHz. The order of the numerator polynomial B(z) is thus defined by the length of the cyclic prefix predetermined by the respective transmission standard, for example M=4.
An approximation of the channel transfer function, for example by means of a rational transfer function H(z) by using mathematical optimization methods, for example the method of least error squares, disadvantageously achieves a global optimum only if the order of the numerator polynomial B(z) and the order of the denominator polynomial A(z) can be selected to be sufficiently large.
In the method according to the prior art it is also disadvantageous that the order of the polynomials is restricted by the length of the cyclic prefix so that secondary minima occur when the channel transfer function of the transmission channel is approximated.